1. Field of the Invention
The present invention relates to a heat exhausting structure and an image forming apparatus, and more particularly, to a heat exhausting structure used in a fixing unit provided in an image forming apparatus.
2. Description of the Related Art
An electrophotographic system is well known as an image forming system. In the electrophotographic system, when an electrostatic latent image formed on a photosensitive member equivalent to a latent image carrier is visualized according to supply of a toner from a developing device, a toner image is transferred onto a recording medium like a recording sheet. The toner image transferred is fixed on the recording medium by melting and permeation actions using heat and pressure in a fixing unit.
The fixing unit heats a recording sheet and fixes an image thereon while holding and conveying the recording sheet using a fixing roller including an internal heat source and a pressure roller. Alternatively, the fixing unit uses a belt wound around rollers to convey a recording sheet. In the belt, unlike the rollers and the like, it is possible to reduce a heat capacity.
When the surface of the belt is heated from the outside rather than the inside of the rollers, it is possible to quicken the rise of a surface temperature of the belt that is in contact with an unfixed toner. In the technology described in “Addition of a Document Copying and Printing Machine using Electromagnetic Induction Heating to the Format Designation” (a material concerning consultation with the Radio Regulatory Council about an amendment of the Radio Law Enforcement Regulations) announced by the Postal Services Agency of the Ministry of Internal Affairs and Communications on Jul. 14, 2000, it is possible to use the electromagnetic induction system as an external heating source.
As devices serving as heat generating sources in the image forming apparatus such as the fixing unit, the image forming apparatus also includes electromagnetic devices like a motor and a clutch and a micro chip or the like used for control. However, in particular, heat from the fixing unit having a large heat capacity may cause an increase in an ambient temperature in the image forming apparatus and exert thermally adverse effect on the devices provided in the image forming apparatus.
For example, since a toner is used as a developer in the developing device, it is likely that coagulation of the toner is caused by a temperature rise in the developing device to make it impossible to perform desired developer supply control. In an optical system, lenses made of resin are often used as optical lenses like an fθ lens. Thus, a regular imaging optical path may change because of thermal deformation or the like to cause a writing failure from which abnormality of an image like color drift occurs.
Thus, conventionally; technologies for discharging heat generated in a fixing unit to the outside are adopted. As an example, in a first conventional technology (Japanese Patent Application Laid-Open No. H11-231760), in general, a heat exhaust fan is arranged near a fixing unit.
In a second conventional technology (Japanese Patent Application Laid-Open No. 2000-98857), an airflow path using a duct is formed between a position near a fixing unit and an outer wall of an image forming apparatus body, a fan is provided on an entrance side of the airflow path, and a cutout, from which the air from the position near the fixing unit can be led in, is formed in a part of the duct to make it possible to lead the hot air in the position near the fixing unit into the duct.
In a third conventional technology (Japanese Patent Application Laid-Open No. 2001-22151), to prevent thermal deformation of optical components, it is proposed to provide a duct that makes it possible to collectively arrange respective optical devices in an airflow path to isolate and radiate heat using the duct.
On the other hand, in a fourth conventional technology (Japanese Patent Application Laid-Open No. 2003-316107), to prevent heat generated in a fixing unit from spreading to a section around the fixing unit, when the fixing unit is arranged near a position where a toner supply tank used for a toner supply unit is set, a heat insulation member is provided between the toner supply tank and the fixing unit or a ventilating unit is provided in addition to the heat insulation member.
In a fifth conventional technology (Japanese Patent Application Laid-Open No 2003-202728), a toner supply tank and a fixing unit are spaced apart from each other.
In recent years, it is desired to reduce time required for staring an image forming apparatus. It is also desired to reduce time for warming-up required for raising temperature of a fixing unit to a predetermined fixing temperature.
Therefore, a heating system for quickly raising temperature to a heating temperature is used in addition to the belt having a small heat capacity. As an example of this heating system, there is the electromagnetic induction heating (IH) system.
In the electromagnetic induction heating system, a metal housing including a magnetic force generating coil is arranged near the surface of the belt to make it possible to heat the belt with radiation heat from the metal housing side that is generated using an eddy current caused by a magnetic line of force transmitted through the metal housing.
However, problems described below occur when the electromagnetic induction heating system is used.
When the belt is heated from the metal housing side near the belt surface, heat retention on a roller side is smaller than heat retention at the time when a heating source for heating the belt is provided on the roller side. Therefore, since heat from the metal housing used for electromagnetic induction heating easily spreads to a section around the metal housing, a temperature rise in a space around the metal housing is caused. As a result, as described above, the heat adversely affects the optical system and the developing device.
On the other hand, insulation performance of the magnetic force generating coil used for electromagnetic induction heating changes according to a temperature rise. The magnetic force generating coil may cause an insulation failure depending on temperature. Thus, it is conceivable to perform heat radiation by airflow as disclosed in the patent documents to prevent a temperature rise in the section around the metal housing and an overheated state of the magnetic force generating coil.
When the airflow is used, a flow rate only has to be increased according to a size of a heat radiation range. However, since an electric current fed to the magnetic force generating coil and a heat value are in a square root relation, to obtain a heat value for reducing a rising edge of warming-up, an electric current suitable for obtaining the heat value is fed. Accordingly, a flow rate of a cooling airflow for controlling the influence of heat on the section around the metal housing has to be increased. As a result, when the flow rate of the cooling airflow is increased, an airflow sound and a driving sound of a fan tend to increase. It is likely that a new problem of environmental noise occurs. In particular, in the inside of the metal housing including the magnetic force generating coil, since a large number of components including not only coils but also structural components like a ferrite are highly densely arranged, a space through which the airflow passes may be small. Consequently, a flow rate of the cooling airflow is secured and a flow velocity for securing this flow rate is increased. Thus, it is likely that noise is noticeably caused.
However, image forming apparatuses in recent years tend to be reduced in size. Therefore, a packaging density of devices in an image forming apparatus is increased. When forced heat exhaust is performed using a fan, there is a problem of airflow in the image forming apparatus as disclosed in the patent document. In other words, simply by setting a suction fan on a wall of the image forming apparatus, airflow for efficient heat exhaust is not caused in some cases. Therefore, there is a deficiency in that the image forming apparatus is filled with heat or ozone cannot be satisfactorily discharged.
As measures against an abnormal temperature rise in the image forming apparatus, there is heat exhaust by airflow generation using a fan or the like. To increase heat exhaust efficiency, it is important to increase a quantity of the air and a velocity (a pressure) of the airflow from the fan and quickly discharge the overheated air to the outside. However, a problem described below occurs when such measures are adopted.
In the image forming apparatus, since writing of an image on a photosensitive member and visualization of an electrostatic latent image formed by the writing are continuously performed, a writing device and a developing device may be arranged relatively close to each other.
Therefore, when a quantity of the air and a velocity of the airflow from the fan are increased, a toner simply adhering to a recording sheet electrostatically at a stage before fixing may be blown off. The toner scattered in the image forming apparatus may enter the writing device. Consequently, since the toner entering the writing apparatus adheres to and soils the optical components, it is likely that an abnormal situation like lack of a part of a written image occurs and an image with a writing failure is obtained.
Thus, in the first conventional technology, to solve the deficiency, the number of fans is increased, a duct having a special structure is provided to form an exhaust flow path, or a plurality of stages of filters is provided.
However, when such a constitution is adopted, it is necessary to collectively arrange respective optical writing devices in the duct used as heat prevention measures for the optical components. Thus, a size of the duct is increased and the duct is required to be arranged not to hinder airflow. As a result, a space for setting components in the image forming apparatus is required. It is likely that a size of the image forming apparatus is increased because of the increase in the space for setting the image forming apparatus.
On the other hand, in the fourth conventional technology, a space for setting the heat insulation member and the ventilating unit in the small space in the image forming apparatus is required. Similarly, in the fifth embodiment, it is necessary to set a relatively large space to prevent heat of the fixing apparatus from affecting the position where the toner supply tank is set. Therefore, the problem concerning the setting space in the image forming apparatus is left unsolved.
When discharge of the air in the image forming apparatus is facilitated to improve heat exhaust efficiency by increasing places where the airflow is generated in the image forming apparatus through addition of the fans, it is likely that an increase in size of the image forming apparatus is caused by an increase in component cost due to the addition of the fans and an increase in the setting space. Moreover, it is likely that driving noise and airflow sounds from the fans are caused more frequently. In particular, when the electromagnetic induction heating system is used as the heating system of the fixing unit, it is likely that noise is caused more noticeably because of a reason described below.
The insulation performance of the magnetic force generating coil used for electromagnetic induction heating changes according to a temperature rise. The magnetic force generating coil may cause an insulation failure depending on temperature. Thus, it is conceivable to perform heat radiation by airflow as disclosed in the patent document to prevent a temperature rise in the section around the metal housing and an overheated state of the magnetic force generating coil.